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The electrical charge and mild acid oxidize the surface of the aluminum, forming
microscopic crystals of aluminum oxide. This oxide surface is very hard, much harder
than the base material, but very thin, often only a thousandth of an inch. While
most aluminums average about 35 to 40 on the Rockwell C scale, the oxide layer is
closer to 52 to 55. A file, usually very hard steel, is often 55 to 58 Rockwell C.
Most automotive-grade ("wet & dry") sandpapers use aluminum oxide grains
as the abrasive. The anodizing prevents further oxidation, so the piece wont rust
or corrode any further, and also makes the surface much more wear and scratch resistant.
The anodic layer is also very porous as it comes out of the bath. Because of this porosity, it will absorb and hold dyes and colors well. The piece is usually colored simply by dipping it in a vat of concentrated dye for several minutes. Once the piece has been dyed, it is sealed, simply by immersing it in clean, boiling water for several more minutes. This process, because of the porosity of the oxide, hydrates the crystalline layer, which swells the oxide, closing the pores, sealing the dye within. Because the layer is so thin, the dye must be heavily concentrated, and even then, the color will be somewhat translucent. This is what gives the color anodizing its characteristic metallic sheen; the shiny base aluminum shows through the color. Because of this semi-transparency, the piece cannot be anodized white; the aluminum would show through, giving a light grey color.
"Splash" or "sponge" patterns are made by first literally slinging a "masking" agent onto the anodized surface, before it is dyed. This prevents the masked areas from accepting the color, so when the mask is removed, silver streaks are revealed underneath. A thickened dye itself can also be 'splashed' or 'sponged' onto the surface, then the part is dyed. Often, the first dye is a much darker color, which leaves, for example, black streaks on a green surface.
A "fade" color can be made by first dying/splashing/sponging the part, then before sealing, the piece is partially lowered into a mild bleach or acid solution, that dilutes or "fades" the dye, just like liquid bleach can fade a pair of pants in the wash. A fade job will, if left in long enough, show a 'clear' or silver color where it spent the most time in the 'fade bath', which slowly darkens to, for example, a light blue where it wasn't in the bath as long, to a medium blue where it was in only briefly, to a deep blue where it was not "faded" at all.
The color you see on the piece is the dye, not technically the anodizing. The
piece can be anodized, but not dyed, (for a shiny silver color) but it cannot be
dyed without anodizing.
The anodic layer can and usually does change the dimensions of the part being anodized. The addition of oxygen to the surface layer makes it thicker, usually adding a half-thousandth or more to the measurement. This doesnt sound like much, but it can make a difference in some finely fit pieces. When a piece is reanodized, like after its been milled and you are having it recolored, or youre changing the dull factory black for a cool splash, the original layer of anodizing must be completely removed down the the unoxidized layer of aluminum beneath. Multiple stripping/reanodizing processes can remove a surprising amount of metal, to the point O-rings might not seal as well, and parts become loose. For example, stripping and reanodizing an aluminum barrel can alter the bore diameter by up to two or three thousandths. Meaning your .688 could become a .691.
Now, there are home methods to anodize aluminum, and one company even sells an anodizing kit. However, I have tried several home recipes, and my results have been poor. I do know people
who have successfully anodized pieces themselves, and dont let me discourage you from trying, but remember, you will most likely spend more money on materials trying to do it yourself, than you would have having a professional plater do the job. Also, home results tend to be fairly low quality, so if youre expecting to do a fancy, shiny splash job, dont bother.
In Figure 2.1 above, we see the original, unanodized aluminum surface on the left, and a greatly magnified diagram of the anodized surface on the right. You can see how the crystals 'grow' out of the surface of the base metal, partially above and partially below the original surface level. This oxide layer is usually about one thousandth of an inch thick, although it can be up to two thousandths. The layer is very dificult to make thicker than that, as the oxide is non-conductive, so once the electrical path through the base metal from the electrolyte is lost, the formation of the layer stops.
In Figure 2.2 above, we see the same anodized layer, after the dye bath. The dye fills the microscopic pores, giving the metal the chosen color. Remember, we're looking at a layer a thousandth of an inch thick, much thinner than even a sheet of paper. The pores are not so much filled with "liquid" dye, rather the very finely-ground dye colorants. Picture the pores as pits filled with "sand grains" instead, with the 'sand' actually bits of the actual pigment elements of the dye.
And finally, here in Figure 2.3, above, we see the surface 'sealed' to trap the dye. Sealing is done by merely boiling the part for a few minutes in clean water. The oxide crystals react with the boiling water by "swelling" and forming smaller crystal nodules, which grab and hold the pigment grains of dye in the pores. So even after washing and drying, the pigment grains are held fast, much like gravel is held in a cement slab.